Firstly, you should probably Bold your section headings, to break the article up and make it easier to read.

SCP 2891

This should have "Item Number:" before it, and should be written as "SCP-2891".

Containment

This section should be titled "Specials Containment Procedures:" (Thats what "SCP" stands for).

kept in a 3 m by 3 m by 3 m

Unless these measurements are critical to containment, you can probably do without. Just say "standard containment cell".

poly-acrylic

"polyacrylic" doesn't have a hyphen.

from all sides, including above and below

"all sides" already includes above and below.

six (6)

You don't need to use this "number (#)" format, it's unnecessary and clutters up the sentence.

You need another line break before the Description.

the potential to absorb and control humanoid and animal individuals

I suggest you change this to something like just "animals".

Under high illumination

Might want to attach a unit to that (e.g. lm or cd/m^2).

The entity, identified as

This would work better if it was something like "This entity, hereby referred to as".

falls below a certain threshold

Which is?

fade out of existence over the course of 1-10 minutes

So what happens to something in the ninth minute of it's 10 minute disappearance.

that have a change

Guessing you meant "chance".

as no way has been determined to retrieve them

Er…except shining a light on the crystal, and that's not really what "terminated" means.

generally refusing to answer questions

Just generally? So what questions has it answered?

Experiment 2891-1

This test is basically just "yep, this thing sure does what it says it does." and I don't think it adds anything to the piece.

Threshold identified at 75 W/cm

This number should've been way up in the description, in my opinion.

There's not really much to this article, as it is now. It's a crystal that absorbs people when it's dark, and then spits them out angry and crazy when it's bright. Ok. Why? Where did it come from? How did The Foundation get it? What does it seem to want? This piece needs backstory, or something else unique and interesting to make it stand out from the hundreds of other "magic items what make you go crazy".

Here's an idea I have for expansion on this, then. There is already established a personality linked directly with the crystal itself. Would writing a more interesting characterization than "angry and abusive" into this personality help to add interest and uniqueness to this item?

Alright, another idea on top of deepening the personality. In experiments, I had listed a humanoid SCP terminated to end a containment breach. Termination no longer fits with the feel, so I either delete the referenced SCP entirely, or incorporate it more fully into the current SCP (and potentially have some fodder for another SCP). Would incorporation of this other SCP be a reasonable possibility?

"The ability to absorb and control both humans and animals" immediately stands out as pretty unprofessional wording. In fact… I'm not even sure what this specifically does at all. Some body swapping thing going on? The crystal absorbs people and turns into people?